It has been observed that a vehicle with manual transmission (MT) experiences a vibration back and forth when sudden acceleration or deceleration is exerted on the vehicle. This phenomenon is explained in the following manner. The traction wheels of vehicle cannot respond to a sudden variation of engine torque, thereby causing a torsional vibration for a drive line transmitting torque from an engine to the traction wheels, specifically a drive shaft (drive axis). This torsional vibration causes a vibration of rotational velocity of the engine, developing a vibration in the driving force of vehicle.
As shown in FIG. 1A, a drive shaft is modeled by a combination of spring and damper in parallel. If the drive shaft twists as a result of a sudden variation of engine torque, it stars a torsional vibration with spring effect. The amplitude of this torsional vibration becomes smaller with the passing of time by damper effect. Therefore, a model of simple harmonic motion, the amplitude of which gradually attenuates, is representative of the torsional vibration of drive shaft.
The torsional vibration (torque variation) of a drive shaft caused by acceleration or deceleration of a vehicle will be described referring to FIG. 1B. In FIG. 1B, the vertical axis represents the rotational velocity of an engine which has an effect on the torsional vibration of a drive shaft and the horizontal axis represents time. When the acceleration of vehicle is suddenly started from the condition of constant rotational velocity of engine, the rotational velocity of engine should linearly increase along the broken line shown in the figure on the condition that a torsional vibration of drive shaft does not occur. However, actually the rotational velocity of engine increases in a vibratory manner as the torsional vibration of drive shaft occurs. Occupants of the vehicle feel this vibration of rotational velocity of engine as a torque variation, which makes them feel swinging back and forth and experience some uncomfortable feeling accordingly. This phenomenon also occurs in a vehicle driven by a motor.
In order to solve the problem, such a method as delaying the response of an engine to an accelerator pedal by delaying the ignition timing of engine so as to decrease the engine power while the accelerator pedal is depressed has been generally practiced. However, the method has not yet solved the risk that the performance of vehicular acceleration is sacrificed.
This problem also occurs in a hybrid vehicle having an engine and a motor as driving means. Japanese Published Patent Application 2001-57714 discloses a method aiming at solving of the problem for such a type of vehicle. When a command for a sudden torque variation is generated, the command is not directly transmitted to a drive line but undergoes a delay process, and thereby the torque is gradually varied and the vibration of a drive shaft is restricted. However, this method has not solved the problem that the performance of vehicular acceleration is degraded as a result of the poorer response of motor to the command for torque variation.
Further, the invention disclosed in Japanese Published Patent Application 2001-28809 is named, which is an improvement of Japanese Published Patent Application 2001-57714.
When a command for torque variation of a motor is generated, the torque actually powered by the motor, which is estimated with an inverse model based upon a rotational velocity of motor, is defined as estimated torque. Control torque for controlling a torque vibration is then computed based upon the difference between this estimated torque and that requested by the command. The summation of the control torque and that requested by the command is subsequently delivered to the motor. However, this method has difficulty in achieving highly accurate control of output torque of the engine for a hybrid vehicle employing an engine and electric motor in parallel. In this way, an error in engine torque is added to the estimated torque, thereby resulting in the generation of an inappropriate compensation torque.